The present invention relates generally to the construction of a monopolar membrane electrolytic cell for the production of chlorine, alkali metal hydroxides and hydrogen, wherein each electrolytic cell unit has at least one central electrode assembly with at least two end electrode assemblies on either side thereof so as to form a closed system for efficient utilization of the materials for the central electrode assemblies. More particularly the present disclosure relates to an improved electrolytic cell structure having a central electrode assembly with an end electrode assembly contained on either side thereof to form a closed cell such that when several of the cells are linked in series or parallel to form an electrolytic cell bank, any given cell may be removed therefrom without interruption of production from other identical cell units. This employs the use of the planar electrode elements such that a planar membrane may be spaced between the elements to provide a membrane electrolytic cell especially suitable for the production of chlorine, caustic (sodium hydroxide) and hydrogen.
Chlorine and caustic are essential and large volume commodities which are basic chemicals required in all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of alkali metal chlorides with a major portion of such production coming from diaphragm type electrolytic cells. In the diaphragm electrolytic cell process, brine (sodium chloride solution) is fed continuously to the anode compartment and flows through a diaphragm usually made of asbestos, backed by a cathode. To minimize back migration of the hydroxide ions, the flow rate is always maintained in excess of the conversion rate so that the resulting catholyte solution has unused alkali metal chloride present. The hydrogen ions are discharged from the solution at the cathode in the form of hydrogen gas. The catholyte solution, containing caustic soda (sodium hydroxide), unreacted sodium chloride and other impurities, must then be concentrated and purified to obtain a marketable sodium hydroxide commodity and sodium chloride which can be reused in the chlorine and caustic electrolytic cell for further production of sodium hydroxide.
With the advent of technological advances such as the dimensionally stable anode and various coating compositions therefor which permit ever narrowing gaps between the electrodes, the electrolytic cell has become more efficient in that the current efficiency is greatly enhanced by the use of these electrodes. Also, the hydraulically impermeable membrane has added a great deal to the use of electrolytic cells in terms of the selective migration of various ions across the membrane so as to exclude contaminents from the resultant product thereby eliminating some costly purification and concentration steps of processing.
The dimensionally stable anode is today being used by a large number of chlorine and caustic producers but the extensive commercial use of hydraulically impermeable membranes has yet to be realized. This is at least in part due to the fact that a good electrolytic cell structure for use of the planar membrane versus the three dimensional diaphragm has yet to be provided. The geometry of the diaphram electrolytic cells structure makes it undesirable to place a planar membrane between the electrodes, hence the filter press electrolytic cell structure has been proposed as an alternative electrolytic cell structure for the use of membrane in the production of chlorine, alkali metal hydroxides and hydrogen.
A bipolar filter press electrolytic cell is a cell consisting of several units in series as in a filter press in which each electrode except the two end electrodes act as an anode on one side and a cathode on the other, with the space between these bipolar electrodes being divided into an anode and a cathode compartment by the membrane. In a typical operation, an alkali metal halide is fed into the anode compartment where halogen gas is generated at the anode. Alkali metal ions are selectively transported through the membrane into the cathode compartment and combined with hydroxide ions at the cathode to form alkali metal hydroxides and liberate hydrogen. In this type of cell the resultant alkali metal hydroxide is significantly purer and more concentrated, thus minimizing an expensive salt recovery step of processing. Cells where the bipolar electrodes and diaphragms or membranes are sandwiched into a filter press type construction may be electrically connected in series, with the anode of one connected to the cathode of an adjoining cell through a common structure member of some sort. This arrangement is generally known as a bipolar configuration.
While the filter press electrolytic cell provides certain economies in operation with the use of a membrane there still remains the problem that if a given cell section within the cell goes bad, the entire cell structure must be broken down in order to remove the faulty component and the entire cell is out of production for the given period of time. Furthermore, hydrogen embrittlement poses a materials problem for the bipolar configuration. Therefore, it would be exceedingly advantageous to develop a membrane electrolytic cell unit which may be taken out of an electrolytic cell bank without having to discontinue production of the entire electrolytic cell bank.